CA2088393A1 - Method for stabilizing metals in wastewater sludge - Google Patents

Abstract

ABSTRACT

A method for treating the sludge from industrial wastewater streams which contain soluble metals by adding to the sludge an effective amount of a metal complexing agent sufficient to inhibit leaching of the metal from the sludge solids into the environment. The metal complexing agents comprise diethylcar-bamate, dimethyldithiocarbamate, sodium trithiocarbonate the salts thereof or tolytriazole.

Description

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METHOD FOR STABILIZING METALS IN WASTEWATER SLUDGE

FIELD OF THE INVENTION

The present invention relates to the treatment of sludge produced from wastewater streams. Specifically, it deals with the stabilization of the precipitated metals found in the wastewater sludge.

BACKGROUND OF THE INVENTION

Metal-bearing wastewaters produced in industry have traditionally been treated by the addition of lime (calcium hydroxide) or caustic (sodium hydroxide) to precipitate metal hydroxide solids out of solution. The resulting solids are then separated from the clear water by settling or flotation.
Typically, polymer treatments are added to the waste treatment system to aid in flocculating and agglomerating the solids for easier removal and less carryover in the effluent. The agglo-merated solids are then collected as sludge, which is often disposed of by landfilling. Large amounts of these sludges are produced daily as a waste product from many different types of manufacturing processes involving metal fabrication, plating, finishing, etc.

2~5~3 However, as a result of the enactment of the Resource Conservation Recovery Act (RCRA), disposal of certain solid wastes recently became subject to the EPA Toxicity Character-istic Leaching Procedure (TCLP) test (Test Method 1311, Federal Register, March 29, 1990 revised June 29, 1990 herein incorpo-rated by reference). Sludges produced from treatments in com-mercial metals-removal applications are classified as solid wastes and are thus subject to this test, which serves as one of the criteria for distinguishing between hazardous and non-hazardous wastes.

This test requires acidification of the sample, followed by instrumental analysis to measure the concentrations of any metallic ion(s) that may have been leached from the sample.
Metal hydroxide-based sludges possess an inherent disadvantage with respect to this test method, since metal hydroxides exhibit resolubilization behavior under suboptimal pH conditions, including those of the TCLP test protocol.

Other treatments for metals removal also produce solid metal-containing precipitates. Precipitation of metal sulfides via addition of a soluble sulfide source (for example, sodium sulfide) is an effective technique for soluble metals removal.
However, this process has a disadvantage in that the precipi-tated metal sulfide solids contained in the resulting sludge are easily oxidized to metal sulfates, and are then resolubilized, since many metal sulfate salts are water soluble.

~ f~ 3 3 The use of sodium borohydride to chemically reduce soluble metals to their elemental forms has also been used. This process produces very low volumes of sludge solids consisting of solid elemental metals. However, these metal solids will easily redis-solve (i.e., corrode) if left in contact with an aqueous system.

Thus, each of the metal-based sludges generated from the techniques described above are likely to be relatively unstable under conditions similar to those of the TCLP test.

RELEVANT ART

Much of the art within the broad field of wastewater treatment focuses on treating the wastewater stream to remove the metals contained in the effluent. Known metal treating agents, such as alkali metal trithiocarbonates and alkyl thiocarbamates, are added to the wastewater stream in order to complex with both water soluble and suspended insoluble metals. This complex forms a precipitate and can be flocculated and agglomerated by further treatment.

U.S. Patent No. 4,612,125, discloses such a method.
Sodium trithiocarbonate is added to the metal containing waste-water. The metal-tri-thiocarbonate complex is then filtered out of the effluent, leaving a sludge cake relatively free of soluble trace metals.

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Similar to the '125 patent is U.S. Patent No. 4,678,584 which discloses the use of a trithiocarbonate to remove heavy metals from not only wastewater streams but from other metal bearing liquid, such as oil to be recycled as well. As in the previous patent, a metal-trithiocarbonate precipitate is formed which is then separated and removed from the liquid.

The same metal complexing agent is also utilized in U.S.
Patent No. 4,943,377. The sodium polythiocarbonate compound is added to the industrial wastewater or waste oil stream to complex with the dissolved heavy metals contained in the stream. The resulting precipitate is then separated from the liquid by gravity settling or removed by filtration.

The Chemical Abstracts contain a reference to a Japanese - Kokai published in 19893 CA 112:204187g, which discloses treating the waste solids directly in order to prevent leaching of the metals contained therein. The treatment compound is a reaction product of ethylene diamine, sodium monochloroacetate and carbon disulfide.

U.S. Patent No. 4,264,436 discloses the use of a triazole compound to remove soluble metal salts from turbine/circulating oils. Metals specifically treated are copper, iron and lead. The resulting sediment is then filtered or otherwise removed from the oil .

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Triazole, such as the alkyl substituted benzotriazole disclosed in U.S. Patent 4,744,950, are well known as corrosion inhibitors for copper bearing metallurgies. This patent teaches the utility of these compounds in aqueous cooling systems.

DETAILED DESCRlPtION OF THE INVENTION

The present invention comprises the addition of a metal complex;ng agent to the sludge extracted from the wastewater stream of various industrial processes. The metal complexing agents encompassed by the invention are dimethyldithiocarbamate (DTC), diethyl carbamate (DEC) trithiocarbonate (TTC), the salts thereof, and tolyltriazole (TTA)..

The effluent wastewater stream is traditionally treated in a clarifier with either caustic (NaOH) or lime (Ca(OH)2). The amount of caustic or lime added to the wastewater stream will vary depending upon the initial, pretreatment pH of the water. The objective, though, is to raise the pH to a value corresponding to the minimum solubility of the resulting hydroxide solids generated from the chemical precipitation reaction of the metal iun(s) and free hydroxyl ion. The desired pH is typically around 9.

Once in the clarifier, the lime or caustic will complex with the soluble metals contained in the effluent wastewater to form metal hydroxides. These metal hydroxides will then precipi-tate out of solution and drop to the bottom of the clarifier.

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This precipitate is periodically drawn off from the bottom of the clarifier to form a sludge. The amount of solids present in the sludge will depend upon numerous variables, such as the amount of soluble metals present in the pretreated wastewater and the frequency of sludge removal. Many metal bearing sludges will contain up to about 30% by weight solids, with most of them having about 20% by weight or less.

The sludge drawn off from the clarifier is then typically transported to a press for dewatering. Prior to pressing, however, is the most desirable stage to add the metal complexing agents of the present invention. These compounds are added to the sludge in an amount sufficient to stabilize the metal hydroxides contained in the sludge solids. Stability is defined herein as a reduction in the propensity of the metal hydroxide in the dewatered sludge to redissolve and leach out into the environment as solubilized metals or ions, under the acidic conditions which can be found in the landfill or other places of disposition for the dewatered sludge. Stabilization of these metal hydroxides will permit the sludge to pass the stringent requirements of the TCLP test and satisfy the mandate of the RCRA regarding the disposal of metals containing solid wastes.

The maximum acceptable levels for certain metals, as defined in the Federal Register (Toxicity Characteristic Final Rule, March 29, 1990) are as follows:

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TCLP Maximum Limits for Inorganics DPm Arsenic 5 Barium 100 Cadmium Chrom;um 5 Lead 5 Mercury 0.2 Selenium Silver 5 In the future, TCLP maximum limits for metals may become even more stringent. Additionally, maximum lim;ts may be defined for more metals not presently listed.

The amount of the metal complexing agent according to the present invention which is added to the sludge is in the range of 5 to 50,000 ppm. The most appropriate amount of treatment agent necessary will vary from process to process depending on the amount and type of solids present in the sludge and the prepara-t;on of metal hydroxides present in the sol;d waste. The proper administration level should be determined by running period;c TCLP
tests.

EXAMPLES

The follow;ng examples show the util;ty of the present invention. There is no intention to limit its scope to these results.

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Example 1 A sample of waste pickle liquor was obtained from a specialty steel mill located in Reading, PA. The pickle liquor contained hydrochloric and sulfuric acids, as well as large amounts of dissolved metals (see Table I, below) and relatively small amounts of alkaline cleaner waste.

This steel mill currently treats this waste pickle liquor by adjustments with lime to a pH of 9, followed by addition of an acrylamide/acryl;c acid anionic polymer as a settling aid. After solids settling, the mixed metal hydroxide sludge is dewatered using a belt press.

For the purpose of facilitating handling, the waste pickle liquor was diluted ten-fold and adjusted to a pH of 9. NaOH and Ca(OH)2 were used as benchmark standards. Other test samples were treated first with lime (to pH 9) and then with two different concentrations of each of the sodium salts (Na) of DTC and TTC.
The sludges were then analyzed according to the TCLP protocol.
Results are shown in Table II.

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TABLE I
Analysis of Steel M;ll Waste Acid Pickle Liquor (prior to treatment) -Component Concentration(ppm) Al 6.8 Sb 0-34 As 0.15 Ba 1.87 Cd 0-09 Cr 7 49 Co 4.7 Cu 27.0 Fe 2166 Pb 1.5 Mn 19.1 Hg < 0.002 Ni 210 Se < 0.025 Zn 6.5 Note: This sample was diluted ten-fold and filtered through a 0.45 um filter.

2~ 3~3 --lo--TABLE II
Effect of Chemical Treatments on TCLP Sludge Extract Analyses Wastewater Substrate: Steel Mill Waste Acid Pickle Liquor Sludqe Treatment Lime & Lime & Lime & Lime &
Sodium NaDTC NaDTC NaTTC NaTTC
Element Hydroxide Lime 25 ppm 250 ppm 25 ppm 250 ppm Al, ppm 4.70 4.75 2.82 4.25 3.99 1.78 Sb, ppm 1.40< 0.02 < 0.02 < 0.02 < 0.02 < 0.02 As, ppm 0.2400.04000 0.02500 0.04700 0.01700 0.02500 Cd, ppm < 0.00200< 0.00200 < 0.00300 < 0.00200 < 0.00200 < 0.00200 Cr, ppm 131. 30.4 14.3 21.3 22.1 12.6 Co, ppm 2.10 1.70 2.18 2.11 2.00 1.92 Cu, ppm 11.5 9.04 6.75 6.71 5.68 0.344 Fe, ppm 813. 184. 110. 134. 138. 447.
Mn, ppm 8.60 10.1 12.6 12.0 11.1 10.4 Hg, ppm 0.00020< 0.0 < 0.0 < 0.0 < 0.0 < 0.0 Hi, ppm 99.0 71.5 90.1 84.0 84.0 84.2 Se, ppm < 0.02000< 0.02 < 0.02 < 0.02 < 0.02 < 0.02 Zn, ppm 1.900.766 0.915 0.815 0.793 0.715 Table II shows that the TCLP extracts obtained from the sludge treated with NaDTC and NaTTC after lime adjustment contained lower concentrations of several dissolved metals compared to the sludge sample treated with lime only. Final metal concentrations were determined by ICP [Inductively Coupled Plasma].

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EXAMPLES

ExamDle 2 A sample of waste pickle liquor was obtained from a specialty steel mill located in Reading, PA. The pickle liquor contained hydrochloric and sulfuric acids, as well as large amounts of dissolved metals (see Table Il, below) and relatively small amounts of alkaline cleaner waste.

This steel mill currently treats this waste pickle liquor by adjustments with lime to a pH of 9, followed by addition of an acrylamide/acrylic acid anionic polymer as a settling aid. After solids settling, the mixed metal hydroxide sludge is dewatered using a belt press.

For the purpose of facilitating handling, the waste pickle liquor was diluted tenfold and adjusted to a pH of 9.
NaOH and Ca(OH)2 were used as benchmark standards. Other test samples were treated with tolyltriazole. The sludges were thèn analyzed according to the TCLP protocol. Results are shown in Table II.

TABLE Ill Analysis of Steel Mill Waste Acid Pickle Liquor (prior to treatment) Component Concentration(PPm) Al 6.8 Sb 0-34 As 0 15 Ba 1.87 Cd 0-09 Cr . 7.49 Co 4.7 Cu 27.0 Fe 2166 . Pb 1-5 Mn 19.1 Hg < 0.002 Ni 210 Se < 0.025 Zn 6.5 0 Note: This sample was diluted ten-fold and filtered through a 0.45 um filter.

TABLE IV
Effect of Chemical Treatments on TCLP Sludge Extract Analyses Wastewater Substrate: Steel Mill Waste Acid Pickle Liquor Sludqe Treatment Sodium TTA
Element Hvdroxide Lime 50 Dpm Al, ppm 4.70 4.75 < 0.30 Sb, ppm 1.40 < 0.02 < .15 As, ppm 0.240 0.04000 < .2 Cd, ppm < 0.00200 < 0.00200 < .02 Cr, ppm 131. 30.4 0.05 Co, ppm 2.10 1.70 < 0.03 Cu, ppm 11.5 9.04 < 0.03 Fe, ppm 813. 184. 0.16 Mn, ppm 8.60 10.1 1.10 Hg, ppm 0.00020 < 0.0 < 0.0002 Ni, ppm 99.0 71.5 < 0.09 Se, ppm < 0.02000 < 0.02 < 0.02 Zn, ppm 1.90 0.766 < 0.05 pb, ppm 0.07 0.032 < 0.02 Table IV shows that the TCLP extracts obtained from the sludge treated with TTA contained lower concentrations of several dissolved metals compared to the sludge samples treated with caustic only. Final concentrations were determined by ICP
[Inductively Coupled Plasma].

Exam~le 3:

There are instances when a producer of wastewater and wastewater sludge has a problem with high concentrations of specific metals. The following example shows the value of TTA

2 ~ $ ~i as a sludge stabilizer after treatment with caustic. At a southern manufacturing facility, cooling tower blowdown water was obtained where cadmium levels exceeded NPDES (National Pollutant Discharge Elimination System) standards.

The substrate samples as received from the source contained 0.272 ppm of cadmium. This is in excess of the NPDES
limit of 0.05 ppm.

The samples were first treated with sodium hydroxide to adjust pH to approximately 9. 100 ppm of TTA was added to the substrate along with 7.5 ml of an acrylic acid/acrylamide copolymer as a flocculant. Results are shown in Table V.

TABLE V
Cadmium Stabilization Usinq TTA
Treatment Amount(Dpm) Cd(vDm) Control (caustic only to pH 9) 0.116 Flocculant polymer 7.5 0.121 TTA + 100 0.06 Flocculant 7.5 Examole 4:

A southeast copper recycling company was experiencing unacceptable levels of nickel in their wastewater effluent and subsequent sludge. The untreated effluent contained 2.79 ppm of nickel. The conventional treatment used by this facility is as follows:

) 50 ppm A12(S04)3 13 H20 as a coagulant 2) pH adjust to 4.0 with H3P04 to free up chelated metals 3) 5 ppm poly (diallyldimethyl ammonium chloride) 4) pH adjust to 8.3 with NaOH to precipitate the metal hydroxides Table VI shows the beneficial effects of adding various amounts of TTA to samples of the substrate already treated as shown above.

TABLE VI
Nickel Stabilization Usina TTA
Treatment Amount(pDm) Ni(ppm) Control (as shown above) 0.82 TTA 5 0.60 TTA 25 0.18 While this invention has been described with respect to ZO particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.

Claims (3)

1. A method for treating wastewater sludge which contain metal hydroxides comprising adding to the sludge a sufficient amount for the purpose of stabilizing the sludge of a metal complexing agent selected from the group consisting of dimethyldithiocarbamate, diethylcarbamate, trithiocarbonate, the salts thereof and tolytriazole.

2. The method of claim 1 wherein the metal complexing agent is added to the sludge in an amount of from 5 to 50,000 ppm.

3. The method of claim 1 further comprising subjecting the sludge to the Toxicity Characteristic Leaching Procedure.